Titanium dioxide photocatalyst containing carbon and method for its production

ABSTRACT

The invetion relates to a titanium dioxide-based photocatalyst containing carbon that is highly photoactive in visible light (vlp-TiO 2 ) and to a method of manufacture. The vlp-TiO 2 is manufactured by mixing a fine grained titanium compount (BET≧50 m 2 /g) with an organic carbon compound and subsequent thermal treatment at temperatures up to 350° C. The carbon content amounts to 0.05 to 4% by weight, preferably 0.4 to 0.8% by weight. The product is characterized by an ESR spectrum which displays only one significant signal in the g value range from 1.97 to 2.05 at g about 2.003. The inventive photocatalyst can be used for to degrade contaminants and pollutants in liquids and gases.

The invention relates to a titanium dioxide-based photocatalystcontaining carbon that is photoactive in the visible range, alsoreferred to as vlp-TiO₂ below.

The invention further relates to a method for producing a titaniumdioxide containing carbon (vlp-TiO₂) that is effective as aphotocatalyst when irradiated with visible light.

Photocatalytic materials are semiconductors in which, when exposed tolight, electron-hole pairs develop, which generate highly reactive freeradicals on the material surface. Titanium dioxide is a semiconductor ofthis kind. It is known that titanium dioxide is capable of removingnatural and artificial contaminants in air and water by irradiation withUV light, in that the atmospheric oxygen is reduced and the contaminantsoxidized (mineralized) into environmentally friendly end products. Inaddition, the surface of titanium dioxide becomes superhydrophilic as aresult of absorbing UV light. This is the basis of the anti-foggingeffect of thin titanium dioxide films on mirrors and windows.

One serious disadvantage of titanium dioxide is the fact that it canonly utilize the UV component of sunlight, i.e. only 3 to 4% of theradiation, and displays either only very weak catalytic activity, ornone at all, in diffuse daylight.

Consequently, attempts have for some time been made to modify titaniumdioxide in such a way that it can also utilize the main component ofphotochemically active sunlight—the visible spectral range betweenroughly 400 nm and roughly 700 nm—to produce the above-mentionedphenomena.

One way of making TiO₂ photocatalytically active in daylight is to dopeit with metal ions, such as V, Pt, Cr or Fe. Another possibility is tocreate oxygen vacancies in the TiO₂ crystal lattice by reduction ofTi⁴⁺. Both developments require complex production techniques, such asion implantation or plasma treatment.

Numerous patents describe nitrogen-modified titanium dioxide that isphotocatalytically active when irradiated in the visible range (e.g. EP1 178 011 A1, EP 1 254 863 A1).

It is furthermore known that modification with carbon increases thephotocatalytic activity of titanium dioxide when irradiated with visiblelight. For example, JP 11333304 A describes a titanium dioxide whosesurface at least partly displays a precipitate of graphite, amorphouscarbon, diamond-like carbon or hydrocarbons. EP 0 997 191 A1 reports ona titanium dioxide with titanium carbide applied to its surface by meansof vapor-phase deposition.

Photocatalytic materials in which titanium dioxide contains, inter alia,nitrogen, sulfur, carbon or other elements as anions, are disclosed inEP 1 205 244 A1 and EP 1 205 245 A1, for example. The anions are said tobe located either at oxygen sites, at interstitial sites or at the grainboundaries of a polycrystalline titanium oxide particle. No informationis given as regards the characterization of the material and aboutcatalytic or physical properties.

The production of titanium dioxide containing 1.0 to 1.7% by weightcarbon from titanium alcoholates by hydrolysis with hydrochloric acidand subsequent heating to 350° C. is also known (C. Lettmann et al.,Applied Catalysis B 32 (2001) 215). In this case, the carbon originatesfrom the ligand of the titanium compound.

According to a further publication, it has been found that hydrolysis oftitanium tetrachloride with tetrabutylammonium hydroxide, followed bycalcination for one hour at 400° C., yields a titanium dioxidepreparation containing 0.42% by weight carbon (S. Sakthivel & H. Kisch,Angew. Chem. Int. Ed. 42 (2003) 4908). In this case, the carbonoriginates from the precipitant and is presumably dispersed relativelyuniformly in the volume (volume doping).

The disadvantage of the known photocatalytic materials is that themethods for producing them are not suitable for industrial-scaleproduction. Either the methods cannot be realized on an industrial scalefor technical reasons, or they would then no longer be economical. Inaddition, most of the products obtained display insufficientphotocatalytic activity in the degradation of pollutants in visiblelight in the range of λ>400 nm, and an only slight light-inducedincrease in hydrophilicity.

Moreover, the products have so far only been optimized in respect oftheir photocatalytic properties. The color and brightness, i.e. theoptical properties, have been disregarded to date. However, use of avery bright vlp-TiO₂ with little inherent color and high photocatalyticactivity has advantages in all applications that tolerate only little orno inherent color of the vlp-TiO₂, such as applications in coatings,specifically in paints and plasters.

The object of the invention is to provide a daylight-active, highlyeffective photocatalyst on the basis of a carbon-modified titaniumdioxide, and to specify an economical method for producing it.

According to the invention, the object is solved by a titanium dioxidecontaining carbon which, compared to pure titanium dioxide, displayssignificant light absorption in the range of λ≧400 nm (vlp-TiO₂) andwhose electron spin resonance (ESR) spectrum measured at a temperatureof 5 K displays only one significant signal in the g value range from1.97 to 2.05.

The object is further solved by a production method in which a titaniumcompound having a specific surface area of at least 50 m²/g according toBET (Brunauer-Emmett-Teller) is mixed intimately with a carboncontaining compound, and the mixture is treated thermally at atemperature of up to 400° C.

Further advantageous developments of the invention are indicated in thesub-claims.

Product

The vlp-TiO₂ according to the invention displays greater photocatalyticactivity than the types described in the prior art. The measure ofphotocatalytic activity (hereinafter referred to as “photoactivity”) isthe degradation of 4-chlorophenol by a defined quantity of vlp-TiO₂during 120-minute irradiation with light having a wavelength λ of ≧455nm. The measuring method is described in detail below. Under thespecified measuring conditions, the photoactivity of the vlp-TiO₂according to the invention is in the region of at least 20%, preferablyin the region of at least 40%, particularly in the region of at least50%.

The carbon content is in the range from 0.05 to 4% by weight, referredto TiO₂, preferably 0.05 to 2.0% by weight, and particularly preferably0.3 to 1.5% by weight. The best results are obtained with carboncontents of 0.4 to 0.8% by weight.

The titanium dioxide particles contain carbon in a surface layer only,and are thus referred to as “carbon-modified” below - as opposed to thevolume-doped titanium dioxide produced according to Sakthivel and Kisch(2003). The carbon or carbon compounds of the vlp-TiO₂ according to theinvention are presumably primarily covalently bonded to the TiO₂ surfacevia oxygen, and alkaline-leachable.

The photocatalyst can additionally contain nitrogen.

In contrast to unmodified TiO₂, the vlp-TiO₂ according to the inventionabsorbs visible light with a wavelength λ≧400 nm. In this context,compared to the value at 400 nm, the Kubelka-Munk function F(R_(∞)),which is proportional to the absorbancy, is roughly 50% at 500 nm androughly 20% at 600 nm.

The electron spin resonance (ESR) spectrum of the vlp-TiO₂ according tothe invention, measured at a temperature of 5 K, is characterized by astrong signal at a g value of 2.002 to 2.004, particularly 2.003. Nofurther signals occur in the g value range from 1.97 to 2.05. Theintensity of the signal at a g value of roughly 2.003 is increased byirradiation with light having a wavelength of λ≧380 nm (UV-free 100 Whalogen lamp, KG5 cold-light filter), compared to the measurement indarkness.

The X-ray photoelectron spectrum (XPS) of the vlp-TiO₂ according to theinvention is characterized by the occurrence of a strong absorption bandat a bond energy of 285.6 eV, referred to the O1s band at 530 eV.

A further characteristic is that, in contrast to the photocatalystaccording to Sakthivel & Kisch (2003), the vlp-TiO₂ does not displaycarbonate bands, either in the X-ray photoelectron spectrum (XPS), or inthe infrared spectrum.

When irradiated with visible light, the vlp-TiO₂ displays a watercontact angle of roughly 8°, whereas unmodified TiO₂ displays a contactangle of roughly 21°.

The new photocatalyst enables pollutant degradation not only usingartificial visible light, but also with diffuse, indoor daylight. It canbe used to degrade contaminants and pollutants in liquids or gases,particularly in water and air.

The photocatalyst can advantageously be applied to various substrates,such as glass (plain and metallized), wood, fibers, ceramics, concrete,building materials, SiO₂, metals, paper and plastics, in the form of athin layer. In combination with its simple production, this thus opensup potential applications in a wide variety of sectors, e.g. in theconstruction, ceramics and automotive industries for self-cleaningsurfaces, or in environmental engineering (air-conditioning equipment,devices for air purification and air sterilization, and in thepurification of water, especially drinking water) as well as forantibacterial and antiviral purposes. The photocatalyst can be used incoatings for interior and exterior purposes, such as paints, plastersand glazes for application to masonry, plaster surfaces, paint coats,wallpapers and wood, metal, glass or ceramic surfaces, or to components,such as composite heat insulation systems and curtain-wall facadeelements, as well as in road surfacings and in plastics, plasticsheeting, fibers and paper. The photocatalyst can moreover be used inthe production of prefabricated concrete elements, concrete pavingstones, roof tiles, ceramics, decorative tiles, wallpapers, fabrics,panels and cladding elements for ceilings and walls in indoor andoutdoor areas.

The light-induced increase in the hydrophilicity of the TiO₂ surfacegives rise to additional applications, such as non-fogging mirrors andwindows in the sanitary sector or in the automotive and constructionindustries.

The photocatalyst is moreover suitable for use in photovoltaic cells andfor photolysis.

The vlp-TiO₂ according to the invention is described in more detailbelow with reference to FIGS. 1 to 11.

FIG. 1 shows the Kubelka-Munk function F(R_(∞)) (arbitrary units), whichis proportional to the relative absorbancy, for unmodified TiO₂ and forC-modified TiO₂ (vlp-TiO₂) as a function of the wavelength, andindicates that, in contrast to unmodified titanium dioxide, the vlp-TiO₂absorbs in the visible spectral range. F(R_(∞)) is roughly 50% at 500 nmand roughly 20% at 600 nm compared to the value at 400 nm.

FIG. 2 shows the electron spin resonance (ESR) spectra of the vlp-TiO₂according to the invention (spectrum A) and of the TiO₂ producedaccording to Sakthivel & Kisch (spectrum B), recorded in darkness at atemperature of 5 K. Spectrum A displays only one significant signal at ag value of 2.003. In addition to the principal signal at a g value ofroughly 2.003, spectrum B displays three further signals in the g valuerange from 1.97 to 2.05.

FIG. 3 contains the X-ray photoelectron spectra (XPS) of the vlp-TiO₂according to the invention (spectrum A) and of the previously known TiO₂according to Sakthivel & Kisch, precipitated from titanium tetrachloridewith tetrabutylammonium hydroxide (spectrum B). The spectrum of thevlp-TiO₂ displays a pronounced C1s signal at a bond energy of 285.6 eV,referred to the O1s absorption band at 530 eV, this indicatingelementary carbon. In contrast, spectrum B displays C1s signals forelementary carbon at a bond energy of 284.5 eV, as well as additionalbands at 289.4 eV and 294.8 eV, these indicating carbonate.Corresponding IR spectra likewise display typical carbonate bands at1738, 1096 and 798 cm⁻¹.

FIG. 4 illustrates the photocatalytic activity of vlp-TiO₂ compared tounmodified TiO₂ in the degradation of 4-chlorophenol (in the form of a2.5×10⁻⁴ molar aqueous solution) by means of artificial visible light(λ≧455 nm). The diagram presents the decrease in the total content oforganic carbon (TOC_(t)) in the solution relative to the starting value(TOC₀). With the vlp-TiO₂, complete degradation is achieved after 3hours.

FIG. 5 illustrates the photocatalytic activity of vlp-TiO₂ compared tounmodified TiO₂ in the degradation of 4-chlorophenol (in the form of a2.5×10⁻⁴ molar aqueous solution) by means of diffuse, indoor daylight.The diagram presents the decrease in the total content of organic carbon(TOC_(t)) in the solution relative to the starting value (TOC₀). Even inlow-intensity, diffuse daylight (7 to 10 W/m² in the range from 400 to1200 nm), the vlp-TiO₂ leads to 80% degradation within six hours.

Even when illuminated by very-low-intensity diffuse daylight (1.6 to <1W/m²), the vlp-TiO₂ still displays significant photoactivity, unlikecommercially available TiO₂ photocatalysts (Degussa P25, KemiraUV-Titan, Sachtleben Hombikat, Tayca MT-100SA). The degradation rate of2.5×10⁻⁴ molar 4-chlorophenol solution was measured as described above.

-   -   a) Light intensity: 1.6 W/m²; duration: 12 h

Catalyst BET surface Degradation rate vlp-TiO₂ 170 m²/g  16% P25 50 m²/g4% UV-Titan 20 m²/g 5% Hombikat 240 m²/g  9% MT-100SA 50 m²/g 5%

-   -   b) Light intensity: <1 W/m²; duration: 24 h

Catalyst BET surface Degradation rate vlp-TiO₂ 170 m²/g 18% Hombikat 240m²/g 3%

FIG. 6 illustrates the photocatalytic activity of vlp-TiO₂ compared tounmodified TiO₂ in the degradation of benzole (5% by volume),acetaldehyde (2% by volume) and carbon monoxide (5% by volume) bydiffuse, indoor daylight. The reaction vessel used is a 1-liter,round-bottom flask, fitted with a filter-paper disk (d=15 cm) coatedwith 12 mg titanium dioxide. The diagram presents the decrease in thetotal content of organic carbon (TOC_(t)) in the atmosphere relative tothe starting value (TOC₀). The curves show the degradation of benzole,acetaldehyde and carbon monoxide by the vlp-TiO₂ according to theinvention, as well as the degradation of acetaldehyde by unmodifiedtitanium dioxide.

FIG. 7 shows an X-ray powder diffractogram of the vlp-TiO₂, whichdisplays only anatase reflections. The crystallite size calculated bythe Scherrer method is in the region of 10 nm.

FIG. 8 shows a photograph of vlp-TiO₂, produced by means ofhigh-resolution transmission electron microscopy (HTEM), with thelattice lines of the crystallites. The crystallite size can be estimatedas having an order of magnitude of 10 nm.

FIG. 9 shows a carbon depth profile of the vlp-TiO₂, presented as theC/Ti ratio. It was determined by means of ion bombardment (Ar⁺) and ESCAanalysis. The indicated bombardment time of 5×10³ seconds corresponds toa depth of roughly 5 nm.

PRODUCTION

The method according to the invention is based on a titanium compound,which is present in the form of an amorphous, semicrystalline orcrystalline titanium oxide or hydrous titanium oxide and/or a titaniumhydrate and/or titanium oxyhydrate, and is referred to below as theparent titanium compound.

The parent titanium compound can, for example, be produced during themanufacture of titanium dioxide, either by the sulfate process or by thechloride process. Titanium hydrate, titanium oxyhydrate or metatitanicacid is, for example, precipitated during hydrolysis of titanyl sulfateor titanyl chloride.

The parent titanium compound can be present in the form of afine-grained solid, or in a suspension of particles with a correspondingsolids content of at least 15% by weight, in which context the specificsurface area of the solid is at least 50 m²/g according to BET,preferably roughly 150 to 350 m²/g according to BET, particularly 150 to250 m²/g according to BET. For economical reasons, titanium hydrate fromthe sulfate process is preferred as the parent titanium compound forindustrial implementation of the method according to the invention. Thistitanium hydrate is advantageously freed of adhering sulfuric acid byprior neutralization and washing, such that the sulfate content of thesolid after drying is <1% by weight, calculated as SO₃.

Organic carbon containing compounds useful for the invention have adecomposition temperature of 400° C. at most, better <350° C.,preferably <300° C. Materials containing carbon, such as wood, carbonblack or activated carbon, and especially hydrocarbons with at least onefunctional group, have proven to be suitable. The functional group canbe: OH; CHO; COOH; NH_(x); SH_(x); COOR in which R is an alkyl or arylresidue. Suitable are—e e.g. succinic acid, glycerol or ethylene glycol.Sugars or other carbohydrates can also be used, as can organoammoniumhydroxides, especially tetraalkylammonium. Mixtures of the compoundsmentioned are also suitable. Preferably, water-soluble polyalcohols witha carbon/oxygen ratio of roughly 0.7 to 1.5, preferably of roughly 1,are used, particularly pentaerythritol. The carbon compound can be usedin the form of a solid, or as a solution, or as a suspension.

The organic carbon compound should have the greatest possible affinityfor the surface of the parent titanium compound, in order to be able toenter into an intimate bond with the latter.

The parent titanium compound is mixed intimately with the organic carboncompound in such a way that the surface of the parent titanium compoundis coated with the carbon compound. In this context, the organic carboncompound can be present on the surface of the parent titanium compoundin physisorbed or chemisorbed form. The surface of the parent titaniumcompound can be coated by dissolution of the carbon compound in thesuspension of the parent titanium compound, or by mixing of thesuspension of the carbon compound with the suspension of the parenttitanium compound. Intensive mixing of the carbon compound with apreviously dried, powdery parent titanium compound is likewise possible.If titanium hydrate is used, the carbon compound can alternatively alsoalready be admixed to the solution to be hydrolyzed during production ofthe titanium hydrate.

In the finished mixture of parent titanium compound and carbon compound,the quantity of carbon compound, referred to the parent titaniumcompound (as solid), is 1 to 40% by weight.

If the finished mixture is present in the form of a suspension, it canbe dried into a powdery solid prior to further processing. Knownmethods, such as spray drying or fluidized-bed drying, are suitable forthis purpose.

The finished and, where appropriate, predried mixture is treatedthermally at temperatures of max. 400° C. The thermal treatment isperformed in an oxidizing atmosphere preferably in air or in a mixtureof oxygen and air. This process results in decomposition of the organiccarbon compound on the surface of the parent titanium compound, and inthe release of H₂O, CO₂ and CO. Although thermal treatment can beperformed in batch mode, e.g. in a commercially available laboratoryfurnace, a continuous process in which a specific temperature profilecan be maintained is preferred for economical reasons. The continuousmethods open to consideration include all methods with which acorresponding temperature profile and the necessary residence time canbe realized. Indirectly and directly heated rotary kilns areparticularly suitable units. Continuously operated fluidized-bedreactors, fluidized-bed driers and heated ploughshare mixers can also beused. The three last-named units can also be operated in batch mode.

Thermal treatment is preferably performed in such a way that a product(vlp-TiO₂) is obtained which has a carbon content of 0.05 to 4.0% byweight, preferably 0.05 to 2.0% by weight, particularly preferably 0.3to 1.5% by weight, and especially 0.4 to 0.8% by weight. A change ofcolor, from white to brown and finally to beige, takes place in thecourse of the thermal treatment. The end product is characterized by abeige to slightly yellowish-brownish color. It is characterized by thefact that the carbon can be detected in amorphous and polycrystallineregions of the surface layer, as well as on the surface itself. Theproduct is photoactive in visible light.

Following thermal treatment, the product is disagglomerated by knownmethods, e.g. in a pin mill, jet mill or opposed jet mill. In the caseof powdery, predried mixtures, thermal treatment mostly leads toagglomerate-free products not requiring further milling. The fineness ofgrind to be achieved depends on the particle size of the parent titaniumcompound. The fineness of grind or specific surface area of the productis only marginally lower, but in the same order of magnitude as that ofthe educt. The targeted fineness of grind of the photocatalyst dependson the field of application of the photocatalyst. It is usually in thesame range as for TiO₂ pigments, although it can also be lower orhigher. The specific surface area according to BET is in the region of100 to 250 m²/g, preferably 130 to 200 m²/g, particularly 130 to 170m²/g.

EXAMPLES

The invention is described more precisely on the basis of the followingexamples, although this is not intended to restrict the scope of theinvention.

Example 1

An aqueous titanium oxyhydrate paste (35% by weight solids), produced bythe sulfate process, is diluted with sufficient distilled water at roomtemperature to obtain a stirrable suspension. The solids content is inthe region of 20 to 25%. NaOH solution (36% by weight) is added until apH value between 6.0 and 7.0 is obtained. The suspension is subsequentlyfiltered and washed with distilled water until the SO₃ content measuredon the dried residue is below 1% by weight.

The titanium oxyhydrate neutralized and washed in this way issubsequently again diluted into a stirrable suspension (25% solids) withdistilled water, and 12% by weight succinic acid, referred to the solid,are added. The succinic acid is added to the suspension in solid form,and the suspension stirred until the succinic acid has dissolvedcompletely. The suspension is heated to roughly 60° C. to improve thesolubility of the succinic acid. While stirring, the suspension preparedin this way is dried under a surface evaporator (IR radiator) to such anextent that a pasty mass is obtained from the suspension. The pasty massis subsequently dried in a laboratory drying oven at 150° C. until thesolids content is >98%.

300 g of the dried titanium oxyhydrate/succinic acid mixture are finelycrushed (e.g. by grinding in a mortar and sieving), and the resultantpowder is placed in a laboratory furnace at 290° C. in a quartz dishwith cover. At intervals of 1 to 2 hours, the quartz dish is removed andthe powder mixed again. After 13 to 15 hours in the laboratory furnace,the color of the powder has changed from initially yellowish togray-black and finally to yellowish-brown. The thermal treatment toobtain vlp-TiO₂ is complete when the carbon content has decreased fromthe initial 5 to 5.5% by weight to roughly 0.65 to 0.80% by weight.

The photocatalyst is subsequently disagglomerated and analyzed todetermine its carbon content, optical properties, BET surface andphotoactivity.

Example 2

Procedure similar to that in Example 1, except that 12% by weightpentaerythritol are added to the titanium oxyhydrate suspension as thesolid.

Example 3

Procedure similar to that in Example 2, except that 5% by weightpentaerythritol are added to the titanium oxyhydrate suspension as thesolid.

Example 4

A titanium oxyhydrate/pentaerythritol suspension containing 5% by weightpentaerythritol is prepared as described in Example 1. As a modificationof Example 1, the thermal treatment of the suspension obtained in thisway is performed in a continuously operated rotary kiln, as follows:

The rotary kiln is operated in counter-current mode and heated directlyby means of a gas burner. The naked flame of the gas burner is protectedby a fire tube, thus preventing direct contact with the product(vlp-TiO₂). The heated kiln length is 7 m, and the inside diameter 0.3m. The suspension is finely sprayed in at the kiln inlet. The suspensionis added at a feed rate of 40 kg/h. Chains installed in the kiln inletensure good turbulence, and thus rapid drying and subsequent comminutionof the dried material. The time required to pass through thecontinuously operated rotary kiln is 1 hour. The kiln temperature in thearea of the outlet is regulated to 260° C. by means of the gas volume atthe burner. The vlp-TiO₂ obtained at the outlet of the kiln has the formof a fine powder with a yellowish-brown color. The vlp-TiO₂ issubsequently disagglomerated in a laboratory mixer (Braun, MX 2050) andanalyzed to determine its carbon content, optical properties, BETsurface and photoactivity.

Example 5

Procedure similar to that in Example 4, except that the kiln temperaturein the area of the outlet is regulated to 280° C. by means of the gasvolume at the burner.

Example 6

A titanium oxyhydrate/pentaerythritol suspension containing 5% by weightpentaerythritol is prepared as described in Example 1. As a modificationof Example 1, the suspension is predried in an electrically heatedfurnace to obtain a powdery solid with a residual moisture content of22%. The thermal treatment of the predried, powdery feed material isperformed in a continuously operated, indirectly heated rotary kiln, asfollows:

The rotary kiln is operated in co-current mode and heated electricallyin three zones. The total heated kiln length is 2,700 mm, and the insidediameter 390 mm. The powdery solid is fed to the kiln inlet via ametering screw. Chains installed over the full length of the rotary kilnensure homogeneous distribution in the kiln and prevent caking on thekiln wall. The feed rate is 25 kg solid per hour. The time required topass through the continuously operated rotary kiln is 0.5 hours. Thekiln temperature is regulated electrically in three heating zones. Thetemperature of each of the three heating zones can be controlledindividually. The vlp-TiO₂ obtained at the outlet of the kiln has theform of a beige-colored, fine powder. The vlp-TiO₂ is subsequentlydisagglomerated in a laboratory mixer (Braun, MX 2050) and analyzed todetermine its carbon content, optical properties, BET surface andphotoactivity.

COMPARATIVE EXAMPLE

A TiO₂ pigment (anatase) with a BET surface of roughly 10 m²/g(commercial product Kronos 1000) is mixed with 12% pentaerythritol andthermally treated as in Example 2.

Thermal Photoactivity treatment Analysis of the vlp-TiO₂ DegradationOrganic Time C content PLV test BET of 4-CP in Example substance ° C.(h) (%) L* b* a* m²/g 120 min. (%) 1 Succinic acid 290 13 0.79 85.4 9.851.63 164 48 2 Pentaerythritol 290 28 0.75 86.9 10.08 1.53 158 50 3Pentaerythritol 290 10 0.76 83.7 10.03 1.59 140 63 4 Pentaerythritol 260*   1** 0.92 85.1 11.7 1.2 152 58 5 Pentaerythritol  280*   1** 0.5085.8 9.4 2.2 160 68 6 Pentaerythritol   300***    0.5** 0.78 83.0 11.02.6 167 86 Compar. Pentaerythritol 290 42 0.82 74.7 9.12 2.50 11.6 <5example *Maximum temperature, measured at the outlet of the rotary kiln**Time required for the feed material to pass through the rotary kiln***Temperature of the three heating zones, measured in the region of theheating elements

The analytical data and photoactivity of the vlp-TiO₂ according to theinvention are compiled in the table.

In addition to good optical values (PLV test), vlp-TiO₂ produced fromtitanium hydrate (Examples 1 to 6) displays excellent photocatalyticactivity in the visible spectral range. The use of an anatase pigmentinstead of titanium hydrate results in a product with no notablephotoactivity (comparative example).

Example 7

5 g titanium dioxide (commercial product TRONOX Titanhydrat-0 fromKerr-McGee Pigments GmbH) are suspended in 20 ml distilled water at roomtemperature, mixed with 5 ml ethylene glycol (commercial product fromFLUKA AG) and treated for 30 minutes in an ultrasonic bath (SonorexSuper RK 106 from Bandelin Electronic, Berlin; 35 kHz, 120 W r.m.s. HFoutput). Following magnetic stirring overnight, the solvent is removed,preferably in a vacuum, the residue dried at 100 to 200° C., preferablyat roughly 200° C., for at least 12 hours, then heated to 300° C. in aclosed vessel within one hour, and subsequently maintained at thistemperature for a further three hours. A change in the color of thepowder, from white to dark-brown and then beige, can be observed in thisprocess. Heating for longer periods results in colorless, inactivepowders.

The elementary analysis of the product yields 2.58% by weight carbon,0.02% by weight nitrogen, and 0.40% by weight hydrogen. Unmodified TiO₂contains 0.07% by weight C, 0.0% by weight N and 0.0% by weight H.

Example 8

To strip the carbon compound on the surface, 5 g vlp-TiO₂ are stirred in100 ml of a 2 M sodium hydroxide solution (pH 12) overnight. Abrownish-yellow extract and a whitish residue with hardly any color areobtained by centrifuging, the latter being dried at 100° C. The powderobtained in this way demonstrates no activity in the degradation of4-chlorophenol in visible light. However, if the powder is combined withthe extract again and heated slightly, preferably to roughly 200° C., itpossesses the same activity in the degradation reaction as the untreated(unleached) vlp-TiO₂.

Example 9

To coat a plastic sheet, a powder produced according to Example 6 issuspended in a liquid, such as methanol or ethanol, in the ultrasonicbath, and the resultant suspension is applied to the sheet as thinly aspossible by means of a spray bottle. Following subsequent drying at 343K, the coating operation can be repeated until the required filmthickness is reached. Instead of the plastic sheet, it is also possibleto use a different substrate, such as paper (see experiment according toFIG. 6) or aluminum (see under Measuring methods h): “dip-coating”).

Measuring Methods

a) Determination of the Optical Values (PLV test) The method is used todetermine the optical values for the brightness L*, tone a* and tone b*of the vlp-TiO₂. Under defined conditions, a powder tablet is producedfrom the vlp-TiO₂ to be tested, using a small hydraulic press fromMATRA, Frankfurt. The HUNTERLAB Tristimulus Colorimeter is subsequentlyused to determine the reflectance values on the powder tablet. Thevlp-TiO₂ is ground prior to production of the tablet. To this end, 100 gof the vlp-TiO₂ obtained are put into a commercially available mixer(manufacturer: Braun; model: MX 2050) and ground 12 times for 5 seconds.After each grinding step, the mixer is opened and the powder thoroughlystirred again. A sheet of white paper, matt on both sides, is placed ona base plate with a circular depression, and a metal ring (height 4 cm,diameter 2.4 cm) is pressed into the depression with the press. Roughly25 g of the ground vlp-TiO₂ are poured into the metal ring, shaking andtapping it gently. The powder is compacted at a pressure of 2 to 3 kN.The pressing operation is performed a second time until the targetedoperating pressure of 15 kN is reached.

The metal ring is carefully turned and pulled to separate it from thebase plate. The paper between the base plate and the ring is removed.The ring now contains the tablet, which is used for the measuringprocedure on the HUNTERLAB colorimeter. The measured values of L*, a*and b* are read off directly on the colorimeter.

b) Determination of the Photoactivity (Pollutant Degradation)

In artificial visible light:

15 mg of the vlp-TiO₂ are dispersed in 15 ml of a 2.5×10⁻⁴ molarsolution of 4-chlorophenol in an ultrasonic bath for 10 minutes, andsubsequently exposed in a water-cooled round cell on an optical bench.Exposure for determining the photoactivity is performed using an OsramXBO 150 W short-arc xenon lamp installed in a focusing lamp housing(AMKO, model A1020, focal length 30 cm). The spectrum of this lamp isillustrated in FIG. 10. The reactions are performed in a 15 mlwater-cooled round cell with an inside diameter of 30 mm and a layerthickness of 20 mm. The reaction suspension can be stirred with alaterally mounted stirrer motor and stirrer magnet. The round cell isillustrated in FIG. 11. The cell is fixed at the focus of the lamp. Thelight is focused in such a way that only the reaction chamber of thecell is irradiated. All components are firmly mounted on an opticalbench. To eliminate UV light, a cut-off filter transparent at λ>455 nm(manufacturer: Schott) is inserted in the beam path. To prevent possibleheating of the reaction chamber as a result of exposure, an IR filter isadditionally mounted in the beam path. This consists of a water-filledcylinder (diameter 6 cm, length 10 cm).

The decline in the 4-chlorophenol concentration is monitored by means ofUV spectroscopy (λ=224 nm) or, in the event of degradation (oxidation),by measuring the total organic carbon content (TOC value).

In diffuse, indoor light:

50 mg of the vlp-TiO₂ are dispersed in 50 ml of a 2.5×10⁻⁴ molarsolution of 4-chlorophenol in an ultrasonic bath for 10 minutes, andsubsequently exposed to indoor daylight in an Erlenmeyer flask (100 ml)while stirring.

Degradation of acetaldehyde gas, benzole vapor and carbon monoxide:

Two round filters, coated with the vlp-TiO₂ on both sides (paper, d=15cm, 12 mg catalyst per filter), are put into a round-bottom flask (1 l)filled with air-saturated acetaldehyde gas (2% by volume), or withbenzole vapor (5% by volume), or with carbon monoxide. The flask issubsequently exposed to daylight in the laboratory, and the decrease inthe pollutants and the formation of carbon dioxide are monitored bymeans of IR spectroscopy.

c) Determination of the Carbon Content

The carbon content is measured as the total organic carbon content(TOC), using the LECO C-200 carbon analyzer. The measuring method isbased on combustion of the organic substance contained in the TiO₂ in aninduction furnace under oxygen gas, and subsequent determination of thecarbon dioxide formed by means of IR detection. The weight of the TiO₂sample is approx. 0.4 g.

d) Determination of the Specific Surface Area According to BET(Brunauer-Emmett-Teller)

The BET surface is measured with a Tristar 3000 from Micromeritics inaccordance with the static volumetric principle.

e) XPS Measurements

The equipment used to measure the bond energies was the Phi 5600 ESCAspectrometer (pass energy of 23.50 eV, Al standard, 300.0 W,

f) ESR Measurements

A Bruker Elexys 580 Spectrometer X-band (9.5 GHz) was used to measurethe electron spin resonance spectra. The sample was evacuated to 10⁻⁵Torr, filled with helium to a pressure of 10⁻² Torr and subsequentlymelted down. The measurement was performed under the followingconditions:

Magnetic field modulated with 100 Hz. RF power: 0.0002 to 1 mW. Field:3340 to 3500 G. Sweep width: 100 to 500 G. Conversion time: 81.92 ms.Time constant: 40.96 ms. Modified amplitude: 0.2 to 13 G, Temperature: 5K. The g value is determined by means of a Hall probe.

g) Measurement of the Diffuse Reflection Spectra (Kubelka-Munk Function)

The diffuse reflection spectra of the powders were measured using aShimadzu UV-2401PC UV/Vis spectrometer equipped with an Ulbricht sphere.The white standard used was barium sulfate, with which the powders wereground in a mortar prior to measurement. The Kubelka-Munk function isproportional to the absorbancy.

h) Superhydrophilic Properties

To measure the contact angle of water, both vlp-TiO₂ and unmodified TiO₂were suspended in distilled water, applied to a 5×5 cm aluminum plate by“dip coating”, and calcined at 400° C. for 1 hour. After storage indaylight, a contact angle of 21° was measured for the unmodifiedtitanium dioxide, in contrast to only 8° for the vlp-TiO₂. The contactangle for the uncoated aluminum plate was 91°.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

1-24. (canceled)
 25. A material, comprising; a photocatalyst based oncarbon containing titanium dioxide particles, wherein the material showssignificant light absorption for light of wavelength λ>400 nm comparedto pure titanium dioxide particles, wherein the carbon content is in therange from 0.05 to 4% by weight, and wherein the carbon content of theTiO₂ particles is only embedded in a surface layer of the TiO₂particles.
 26. The material of claim 25, wherein the Kubelka-Munkfunction F(R∞), which is proportional to the absorbancy, at 500 nm isroughly 50% the value at 400 nm, and at 600 nm is roughly 20% of thevalue at 400 nm.
 27. The material of claim 26, wherein the specificsurface area of the material according to BET is 100 to 250 m²/g. 28.The material of claim 27, wherein the material is characterized by astrong absorption band at a bond energy of 285.6 eV in the X-rayphotoelectron spectrum (XPS), referred to the O(1s) band at 530 eV. 29.The material of claim 25, wherein the Kubelka-Munk function F(R 4),which is proportional to the absorbancy, at 500 nm is roughly 50% thevalue at 400 nm, and at 600 nm is roughly 20% of the value at 400 nm.30. The material of claim 25, wherein the photoactivity is at least 20%.31. The material of claim 30, wherein the photoactivity is at least 40%.32. The material of claim 25, wherein the carbon content is in the rangefrom 0.05 to 2% by weight.
 33. The material of claim 32, wherein thecarbon content is in the range from 0.3 to 1.5% by weight.
 34. Thematerial of claim 33, wherein the carbon content is in the range from0.4 to 0.8% by weight.
 35. The material of claim 25, wherein thespecific surface area of the material according to BET is 100 to 250m²/g.
 36. The material of claim 25, wherein the material does notdisplay significant carbonate bands, either in the X-ray photoelectronspectrum or in the infrared spectrum.
 37. The material of claim 25,wherein the material is incorporated into materials chosen from thegroup consisting of plastics, plastic sheeting, fibers, paper, and roadsurfacing material.
 38. The material of claim 25, wherein the materialis incorporated into construction materials chosen from the groupconsisting of prefabricated concrete elements, concrete paving stones,roof tiles, ceramics, decorative tiles, wallpapers, fabrics, and panelsand cladding elements for ceilings and walls in indoor and outdoorareas.
 39. The material of claim 25, wherein the material isincorporated in devices for air purification and air sterilization andfor water purification especially for antibacterial and antiviralpurposes.
 40. The material of claim 25, wherein the material isincorporated in photovoltaic cells and in devices for photolysis. 41.The material of claim 25, wherein the specific surface of the materialis greater than 100 m²/g.
 42. A material, comprising; a photocatalystbased on carbon containing titanium dioxide particles, wherein thematerial shows significant light absorption for light of wavelengthλ>400 nm compared to pure titanium dioxide particles, and wherein thematerial does not display significant carbonate bands, either in theX-ray photoelectron spectrum or in the infrared spectrum, wherein thecarbon content is in the range from 0.05 to 4% by weight, and whereinthe carbon content of the TiO₂ particles is only embedded in a surfacelayer of the TiO₂ particles.
 41. The material of claim 42, wherein thematerial is characterized by an electron spin resonance (ESR) spectrum,measured at a temperature of 5 K, which displays only one significantsignal in the g value range from 1.97 to 2.05.
 43. The material of claim41, wherein the one significant signal in the ESR spectrum occurs at a gvalue of 2.002 to 2.004.